I don't think Dragons and demons have mantles that are meant to pass on after they die.
Concurred. Otherwise, the number of semi-divine entities controlling whatever it is the Dragons control would remain the same. WOJ is they've been reduced, based on only a few Dragons remaining, and no talk of replacements in the same kind.
I have issues with this even though its a WAG. In the Dresden verse we have evidence that says you are what you eat. I cannot imagine that TWG or TWC would allow deamon power to be absorbed by the swords.
Demon or Fallen? Demons in the DF are basically Wyld Fae animals, from what I understand. Their power, were it swallowed, wouldn't be inherently evil. Fallen power, while corrupted, would originally have come from TWG, so a sword swallowing that would basically be reclaiming and untainting Heaven's power. Not saying I buy the Sword-Devours-Power theory, but I could see it having a "purging" influence, so to speak.
If either of the Queens or Mothers were to become infected, we'd probably see a mission from On High.
Dunno about the Queens. Maeve was a Lady, and no Heaven sanctioned mission was sent that we know of. Now if Mother Winter were nemfected, I could see that kicking off multiple Armageddons and a Ragnarok.